The specific aims of this proposal are to examine the possible interactions of normal, preneoplastic and neoplastic cells in the pathobiology of murine mammary cancer. The underlying assumption, based on extensive evidence in many epithelial cell systems, is that the evolution of the neoplastic cell phenotype occurs as a result of sequential alterations in susceptible cells giving rise to a series of cell phenotypes which are precursors to the neoplastic cell phenotype. Recent in vivo experiments show that the emergence of the neoplastic cell phenotype is influenced by neighboring normal and preneoplastic cells. It is the immediate aim of this proposal to examine the pathobiology of preneoplastic and neoplastic cell emergence in an in vitro system where the three cell phenotypes are cocultured in a common culture vessel. The ability of normal cells to inhibit the growth of preneoplastic and neoplastic cells and the ability of preneoplastic cells to inhibit the growth of neoplastic cells will be assessed by indirect measurements of DNA synthesis, by direct measurements of cell counts, and by intrafat pad transplantation. Several agents, including Selenium, isobutylxanthine, and tetradecanoylphorbol ester, will be used to modulate the interactions of the different cell types. The long-term objective is to determine the cellular basis for cellular interactions which play an important role in the pathobiology of mammary tumorignesis.